U.S. patent number 6,626,947 [Application Number 09/858,411] was granted by the patent office on 2003-09-30 for press fit acetabular cup and associated method for securing the cup to an acetabulum.
This patent grant is currently assigned to DePuy Orthopaedics, Inc.. Invention is credited to Michael C. Jones, James C. Kudrna, Mark B. Lester.
United States Patent |
6,626,947 |
Lester , et al. |
September 30, 2003 |
Press fit acetabular cup and associated method for securing the cup
to an acetabulum
Abstract
An acetabular cup and method of securing the acetabular cup to
an acetabulum so as to provide a bearing surface for a head portion
of a femur is provided. The acetabular cup is shaped to provide a
cementless, press-fit into a reamed acetabulum. The acetabular cup
is formed of a body having a sidewall defining a radius from a
center point of the annular rim to the sidewall wherein the radius
increases in length from the apex to said annular rim. The method
of securing the acetabular cup first includes reaming the
acetabulum with a reamer having a head with a radius of curvature
that is less than the radius from the center point to the annular
rim of the acetabular cup.
Inventors: |
Lester; Mark B. (Warsaw,
IN), Jones; Michael C. (North Webster, IN), Kudrna; James
C. (Lakeforest, IL) |
Assignee: |
DePuy Orthopaedics, Inc.
(Warsaw, IN)
|
Family
ID: |
38255052 |
Appl.
No.: |
09/858,411 |
Filed: |
May 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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678032 |
Oct 3, 2000 |
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Current U.S.
Class: |
623/22.23;
623/22.12; 623/902 |
Current CPC
Class: |
A61F
2/34 (20130101); A61B 17/1666 (20130101); B29K
2995/0041 (20130101); A61F 2002/30403 (20130101); Y10S
623/902 (20130101); A61F 2002/3462 (20130101); A61F
2/30771 (20130101); B29L 2031/7532 (20130101); A61F
2/4609 (20130101); A61F 2002/3403 (20130101); B29K
2023/0683 (20130101); A61F 2220/0025 (20130101); A61F
2002/3054 (20130101); A61F 2002/3495 (20130101); B29K
2995/0053 (20130101); A61F 2002/3456 (20130101) |
Current International
Class: |
A61F
2/32 (20060101); A61F 2/34 (20060101); A61B
17/16 (20060101); A61F 002/32 () |
Field of
Search: |
;623/22.11-22.12,22.21,22.23,22.24,22.32,22.34,22.36,22.38,902
;606/79-81,86,91 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0722973 |
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Jul 1996 |
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EP |
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0729981 |
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Sep 1996 |
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EP |
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WO 99/52474 |
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Oct 1999 |
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WO |
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Other References
Dijkstra, D.J. et al., "Cross-linking of ultra-high molecular
weight polyethylene in the melt by means of electron beam
irradiation", Polymer, Jordan Hill, Oxford, Great Britain, vol. 30,
No. 5, May 1, 1989, pp. 866-873..
|
Primary Examiner: Isabella; David J.
Assistant Examiner: Chattopadhyay; Urmi
Attorney, Agent or Firm: Maginot, Moore & Bowman
Parent Case Text
This application is a continuation-in-part and claims the benefit
of co-pending U.S. patent application Ser. No. 09/678,032 filed
Oct. 3, 2000 entitled Press Fit Acetabular Cup and Associated
Method For Securing the Cup to an Acetabulum.
Claims
What is claimed is:
1. A method of securing an acetabular cup to an acetabulum
comprising: reaming a cavity of a first radius of curvature into an
acetabulum with a reamer, the reamer including a reamer head having
the first radius of curvature; and press fitting an acetabular cup
into the reamed cavity, the acetabular cup having a body defining a
dome having an apex, an annular rim, and an outer sidewall
extending therebetween, wherein said annular rim defines a plane
and has a center point lying in said plane, wherein a radial
distance between said center point and said sidewall gradually
increases from said apex to said annular rim, wherein said radial
distance from said center point to said annular rim defines a first
given length, 1L, wherein said radial distance from said center
point to said apex defines a second given length, 2L, wherein said
second given length is less than said first given length, wherein
said first given length is greater than said first radius of
curvature, and wherein said second given length is approximately
equal to said first radius of curvature.
2. The method of claim 1, further comprising: inserting a liner
into the acetabular cup.
3. The method of claim 2, wherein inserting a liner into the
acetabular cup includes inserting a liner made of a polymeric
material into the acetabular cup.
4. The method of claim 1, wherein 0.5 mm.ltoreq.1L-2L.ltoreq.2.0
mm.
5. The method of claim 1, wherein 1L-2L is approximately 1 mm.
6. The method of claim 1, wherein said acetabular cup comprises a
titanium alloy.
7. The method of claim 6, wherein said titanium alloy comprises
Ti-6Al-4V.
8. The method of claim 1, wherein an outside surface of said body
is porous.
9. The method of claim 8, wherein said porous outside surface is
configured to enhance bone ingrowth.
10. A method of securing an acetabular cup to an acetabulum
comprising: reaming a cavity of a first radius of curvature into an
acetabulum with a reamer, the reamer including a reamer head having
the first radius of curvature; and press fitting an acetabular cup
into the reamed cavity, the acetabular cup being a dome-shaped and
having an apex, an annular rim, and an outer sidewall extending
therebetween, wherein said annular rim defines a plane and has a
center point lying in said plane, wherein a radial distance between
said center point and said sidewall gradually increases from said
apex to said annular rim, wherein said radial distance from said
center point to said annular rim defines a first given length, 1L,
wherein said radial distance from said center point to said apex
defines a second given length, 2L, wherein said second given length
is less than said first given length, and wherein said first given
length is greater than said first radius of curvature.
11. The method of claim 10, wherein 0.5 mm.ltoreq.1L-2L.ltoreq.2.0
mm.
12. The method of claim 11, wherein 1L-2L is approximately 1.0
mm.
13. The method of claim 10, wherein said acetabular cup comprises a
titanium alloy.
14. The method of claim 13, wherein said titanium alloy comprises
Ti-6Al-4V.
15. The method of claim 10, wherein an outside surface of said
acetabular cup is porous.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to an acetabular cup and,
more particularly, to a press-fit acetabular cup and associated
method for securing the cup to an acetabulum.
BACKGROUND OF THE INVENTION
During the lifetime of a patient, it may be necessary to perform a
hip replacement procedure on the patient as a result of, for
example, disease or trauma. The hip replacement procedure may
involve a total hip replacement or a partial hip replacement. In a
total hip replacement procedure, a femoral component having a head
portion is utilized to replace the natural head portion of the
thighbone or femur. The femoral component typically has an
elongated intramedullary stem which is utilized to secure the
femoral component to the patient's femur. In such a total hip
replacement procedure, the natural bearing surface of the
acetabulum is resurfaced or otherwise replaced with a cup-shaped
acetabular component that provides a bearing surface for the head
portion of the femoral component.
Acetabular cups may be secured to the acetabulum in a number of
different manners. For example, acetabular cups may be secured to
the acetabulum by the use of bone cement. However, recent studies
have speculated that it may be desirable to secure artificial
components to natural bone structures without the use of bone
cement. Hence, a number of press fit acetabular cups have been
designed for securement to the acetabulum without the use of bone
cement.
In either case (i.e. cemented or cementless), the acetabulum is
first reamed by the surgeon in order to create a cavity into which
the acetabular cup is secured by the use of a surgical tool known
as a reamer. It is often difficult for the surgeon to properly
match the size of the reamer to the desired acetabular cup
size.
Although press fit acetabular cups have heretofore been referred to
as being "generally hemispherical" in shape, such
heretofore-designed cups, in reality, are sub-hemispherical in
shape. In particular, as shown in the prior art drawing of FIG. 7,
a heretofore designed acetabular cup 100 has an apex or dome 102 at
a proximal end 104 thereof along with an annular rim 106 at a
distal end 108 thereof. In between the dome 102 and the annular rim
106, the prior art acetabular cup 100 has a sidewall that has a
convex proximal surface and a concave distal surface.
However, as shown in FIG. 7, the configuration of the prior art
acetabular cup 100 is sub-hemispherical. In particular, a "true"
hemisphere 114 is shown in FIG. 7 as a phantom line overlay. As can
be seen, a distal face 116 of the annular rim 106 does not, in
fact, lie along the 180.degree. surface (or loosely, the equator
118) of the hemisphere 114, but rather is recessed away from the
equator 118 by a relatively significant distance X. In fact, it is
not uncommon for prior art cup designs to be recessed from the
equator 118 of the cup by as much as 4-5 millimeters (i.e. X=4-5
mm).
Such a configuration has a number of drawbacks associated
therewith. For example, such a large recess distance X (i.e. 4-5
mm) renders it difficult for the surgeon to ream a properly sized
cavity in the acetabulum. In particular, the cutting head of
heretofore-designed reamers are typically configured as relatively
true hemispheres. Hence, when a surgeon reams the patient's
acetabulum, the surgeon has to "estimate" the approximate depth of
the reamed recess. More specifically, if the surgeon reams all the
way to the 180.degree. surface or "equator" of the reamer, the
annular rim 106 of the acetabular cup 100 will be recessed in the
reamed cavity. Conversely, if the surgeon does not ream deeply
enough (i.e. "under reams"), the acetabular cup 100 will not be
fully seated in the reamed cavity of the acetabulum. In light of
the fact that surgeons occasionally select a reamer that is
slightly smaller in size than the acetabular cup to be implanted,
under reaming may also disadvantageously lead to bone fracture of
the acetabulum since excessive force is often utilized to insert
the cup into the undersized (i.e. under reamed) cavity. Some of the
early bone cemented cups did not suffer from this problem by being
configured more closely as "true" hemispheres. However, as
indicated above, such cups undesirably required the use of bone
cement during implantation thereof.
Another drawback associated with heretofore-designed press fit
acetabular cups relates to the configuration of the outer shell. In
particular, in an attempt to increase retaining forces, a number of
acetabular cups have been designed with a flared rim (known as dual
radius or "bubble" cups) or a frusto-conically shaped annular rim
portion (known as dual-geometry cups). Although the configuration
of such cups may generate relatively strong retention forces at the
rim portion of the cup, surface contact and therefore retention
forces are relatively small at the portions of the outer shell
other than the rim portion, particularly in the dome area.
Moreover, such reduced surface contact at the portions of the outer
shell other than the rim portion reduces bone ingrowth in such
portions.
With the above-mentioned heretofore-designed press-fit acetabular
cups, a two-part reaming process is typically necessary. The
two-part reaming process involves reaming of the acetabulum using a
reamer of a first size, then reaming the acetabulum using a reamer
of a second size. The more reaming, the more likely that a problem
will occur. For example, many conventional cementless acetabular
cup systems use a cup that is two millimeters (2 mm) larger than
the last reamer size used. Inserting this size cup into the
undersized reamed acetabulum to accommodate this system is
sometimes difficult, particularly with resistance in the dome area
of the cup, which is also larger than the last reamer size
used.
What is needed therefore is an acetabular cup and associated method
that overcomes one or more of the above-mentioned drawbacks.
What is further needed is an acetabular cup and associated implant
method that allows for the cup to be secured to the acetabulum
without the use of bone cement.
What is also needed is an acetabular cup and associated implant
method that facilitates greater amounts of bone ingrowth relative
to heretofore designed acetabular cups.
What is still further needed is a cementless acetabular cup that
utilizes a single reaming process for implant.
SUMMARY OF THE INVENTION
The present invention is a press-fit acetabular cup and method of
securing the acetabular cup to an acetabulum so as to provide a
bearing surface for a head portion of a femur. The acetabular cup
is shaped to provide a cementless, press-fit into a reamed
acetabulum. The acetabular cup is formed of a body having a
sidewall defining a radius from a center point of the annular rim
to the sidewall wherein the radius increases in length from the
apex to said annular rim. The method of securing the acetabular cup
first includes reaming the acetabulum with a reamer having a head
with a radius of curvature that is less than the radius from the
center point to the annular rim of the acetabular cup.
According to one embodiment of the present invention, there is
provided an acetabular cup. The acetabular cup includes a body
defining a dome having an apex and an annular rim. The dome is
defined by an increasing radius sidewall that extends from the apex
to the annular rim. The annular rim defines a plane having a center
point. Wherein a radius from the center point to the annular rim
has a first given length, 1L , the center point to the apex
defining a second given length, 2L, and the second given length is
less than the first given length.
According to another embodiment of the present invention there is
provided an acetabular cup. The acetabular cup includes a
dome-shaped shell having an apex and an annular rim. The
dome-shaped shell has a sidewall defining a radius from a center
point of the annular rim to the sidewall, wherein the radius
increases in length from the apex to the annular rim.
According to yet another embodiment of the present invention, there
is provided a method of securing an acetabular cup to an
acetabulum. The method includes reaming a cavity of a first radius
of curvature into an acetabulum with a reamer, the reamer including
a reamer head having the first radius of curvature, and press
fitting an acetabular cup into the reamed cavity, the acetabular
cup having a body defining a dome having an apex and an annular
rim, the dome defined by a gradually increasing radius sidewall
that extends from the apex to the annular rim, the annular rim
defining a plane having a center point, wherein a radius from the
center point to the annular rim has a first given length, 1L, the
center point to the apex defining a second given length, 2L, the
second given length is less than the first given length, the first
given length is greater than the first radius of curvature, and the
second given length is approximately equal to the first radius of
curvature.
According to a further embodiment of the present invention, there
is provided a method of securing an acetabular cup to an
acetabulum. The method includes reaming a cavity of a first radius
of curvature into an acetabulum with a reamer, the reamer including
a reamer head having the first radius of curvature, and press
fitting an acetabular cup into the reamed cavity, the acetabular
cup having a dome-shaped shell having an apex and an annular rim,
the dome-shaped shell having a sidewall defining a shell radius
from a center point of the annular rim to the sidewall, wherein the
shell radius gradually increases in length from the apex to the
annular rim, and the shell radius at the annular rim is greater
than the radius of curvature.
The present cementless acetabular cup provides an enhanced
peripheral press-fit with reduced loading in the dome area. This
helps provide initial stability for bone ingrowth and long-term
fixation. Further, a good peripheral fit may help protect against
the migration of wear debris from a "pumping action" of the
effective joint space.
Additionally, reamer to cup dimensions of the present acetabular
cup have a line-to-line fit in the dome area with a gradual
increase in press-fit or size from the dome to the rim. This allows
the present acetabular cup to achieve maximization of peripheral
contact for stability while minimizing dome loading and helping
reduce the risk of acetabular fracture.
The present acetabular cup also accepts a polyethylene liner or
insert such that the cup and liner achieve congruency therebetween
without rim loading the liner. A positive locking mechanism and
anti-rotation devices in the metal shell/body defining the
acetabular cup secure the liner to the shell. Congruency and secure
locking of the polyethylene liner work together to reduce
micromotion at the shell/liner interface. Further, such high
conformance between the shell and the liner results in efficient
load transfer and reduce contact stresses.
BRIEF DESCRIPTION OF THE DRAWINGS
The various objects, features, and advantages of the present
invention will become apparent and/or better understood by
reference to the following descriptions of the embodiments of the
present invention taken in conjunction with the accompanying
drawing wherein:
FIG. 1 is an exploded perspective view that shows an acetabular cup
and associated bearing insert that incorporate the features of the
present invention therein;
FIG. 2 is an enlarged side elevational view of the acetabular cup
of FIG. 1 with an imaginary true hemisphere superimposed
thereon;
FIG. 3 is a perspective view of a reamer that is utilized to ream
the acetabulum of a patient prior to implantation of the acetabular
cup of FIG. 1;
FIG. 4 is a perspective view of the acetabulum subsequent to
reaming with the reamer of FIG. 3;
FIG. 5 is a view similar to FIG. 4, but showing the acetabular cup
press fit into the cavity reamed into the acetabulum by the
reamer;
FIG. 6 is a diagrammatic view that shows an outline of the
acetabular cup of FIG. 1 superimposed on an outline of the cavity
reamed into the acetabulum by the reamer; and
FIG. 6A is diagrammatic view that shows an alternative embodiment
of an acetabular cup that may be utilized in the prosthetic hip
assembly 10 of FIG. 1;
FIG. 7 is a side elevational view of a prior art acetabular
cup.
DETAILED DESCRIPTION OF THE INVENTION
While the invention is susceptible to various modifications and
alternative forms, a specific embodiment thereof has been shown by
way of example in the drawings and will herein be described in
detail. It should be understood, however, that there is no intent
to limit the invention to the particular form disclosed, but on the
contrary, the intention is to cover all modifications, equivalents,
and alternatives falling within the spirit and scope of the
invention as defined by the appended claims.
Referring now to FIG. 1, there is shown a prosthetic hip assembly
10 for use in either a partial or total hip replacement procedure.
The prosthetic hip assembly 10 includes an acetabular component or
cup 12 and a bearing insert 14. Collectively, the acetabular cup 12
and the bearing insert 14 provide an artificial bearing surface on
which a natural or artificial head portion of a femur (not shown)
may bear. In particular, as shall be discussed in greater detail,
the acetabular cup 12 is implanted into a patient's acetabulum 16
(see FIGS. 4 and 5) such that the bearing insert 14 may then be
positioned in an insert-receiving cavity 18 (see FIG. 1) defined in
the acetabular cup 12. The bearing insert 14 is preferably
constructed from a polymeric material such as polyethylene or
ultra-high molecular weight polypropylene (UHMWPE) thereby
providing a desirable artificial surface on which the head portion
of the femur may bear.
As shown in FIG. 1, the bearing insert 14 has a number of keying
tabs 20 defined therein. The keying tabs 20 are received into a
number of corresponding keying slots 22 defined in the acetabular
cup 12 to prevent rotation of the bearing insert 14 relative to the
acetabular cup 12 when the bearing insert 14 is positioned in the
insert-receiving cavity 18 of the cup 12.
As shown in FIGS. 1 and 2, the acetabular cup 12 includes a cup
body or shell 24 that has a sidewall 26. The body 24 may be made of
any suitable material such as a titanium alloy. One such titanium
alloy is Ti-6Al-4V. The sidewall 26 has a textured or porous outer
surface. Such a textured or porous outer surface enhances bone
ingrowth thereby facilitating long-term attachment of the
acetabular cup 12 to the acetabulum 16. Such a textured or porous
outer surface may be a POROCOAT.RTM. porous coating may be DePuy
Orthopaedics of Warsaw, Ind.
The sidewall 26 extends outwardly at a substantially constant
radius R.sub.C from an apex or dome 28 of the body 24 to an annular
rim 30. In particular, as shown in FIG. 2, an imaginary hemisphere
32 may be superimposed over the acetabular cup 12. The imaginary
hemisphere 32, as with any true hemisphere, possesses an apex 34
and a great circle 36. The great circle 36 is the circle that is
defined by the intersection of the surface of a sphere by a plane
that passes through the center of the sphere. In essence, a sphere
that is bisected along its "equator" into two equal halves forms a
great circle at the plane of bisection. Hence, the center point of
the bisected sphere is the center point of the great circle of the
hemisphere. Accordingly, every point along the surface of the
imaginary hemisphere 32 (and hence every point on the outer surface
of the sidewall 26) lies an equal distance (i.e. the radius
R.sub.C) from a center point 38 of the great circle 36 of the
imaginary hemisphere 32. Indeed, substantially every point on the
sidewall 26 of the cup body 24 is positioned a distance that is
equal to the radius R.sub.C away from the center point 38 of the
great circle 36. It should be appreciated that the textured or
porous outer surface of the sidewall 26 creates a somewhat
irregular or "jagged" outer surface. Hence, as used herein, the
term "sidewall", when utilized in the context of "every point on
the sidewall being positioned a distance equal to the radius (i.e.
R.sub.C) away from the center point of the great circle", is
intended to mean the average or mean height of the jagged outer
surface of the sidewall thereby factoring out any slight
fluctuations in the distance from the center point of the great
circle caused by the textured or porous outer surface of the
sidewall.
The outer face of the annular rim 30 of the cup body 24 defines a
segmental plane 40 (shown as a line in the side elevational view of
FIG. 2) that intersects the imaginary hemisphere 32. The segmental
plane 40 is oriented substantially parallel to the great circle 36
and is spaced apart from the great circle 36 by a relatively small
distance D. Hence, every point on the outer peripheral edge of the
annular rim is spaced apart from the great circle 36 by the
distance D. In one exemplary embodiment, distance D is between 0.5
and 2.0 millimeters. In a more specific embodiment, distance D is
approximately 1 millimeter.
As a result, the cup body 24 of the acetabular cup 12 is configured
as a substantially true hemisphere. Indeed, with the exception of
(1) the portion of the cup 12 near its apex 28 which is removed in
order to facilitate a threaded aperture 42 which is utilized during
implantation of the cup 12, and (2) the portion of the sidewall 26
which would be present if the sidewall 26 was extended the distance
D toward the great circle 32 of the imaginary hemisphere 32, the
cup body 24 is, in fact, configured as a true hemisphere. As shall
be discussed below, such a configuration (i.e. that of a nearly
true hemisphere) provides numerous advantages to the acetabular cup
12 relative to heretofore-designed cups.
Referring now to FIG. 3, there is shown a cutting tool or reamer 50
associated with the prosthetic hip assembly 10. The reamer 50 is
utilized to ream or otherwise cut the acetabulum 16 in order to
form a hemispherically shaped cavity 52 therein (see FIG. 4). The
reamer 50 includes a cutting head 54 secured to a shaft 56. The
cutting head 54 includes a number of cutting projections 58 which
are configured to engage and remove bone material from the
patient's acetabulum 16. The outer edges of the cutting projections
58 define the radius of the hemispherically shaped cutting head 54.
In one exemplary embodiment, the cutting projections 58 of the
cutting head 54 define a true hemisphere. In particular, the
general profile created by the cutting projections 58 (and hence
the cavity created by the reamer 50) is that of a true
hemisphere.
Moreover, the radius of the cutting head 54 is preferably slightly
smaller than the radius R.sub.C of the acetabular cup 12. In one
exemplary embodiment of the present invention, the radius of the
cutting head 54 is between one-half (0.5) and one and one-half
(1.5) millimeters smaller than the radius R.sub.C of the acetabular
cup 12. In a more specific exemplary embodiment of the present
invention, the radius of the cutting head 54 is approximately one
(1) millimeter smaller than the radius R.sub.C of the acetabular
cup 12. For example, if the anatomy of a given patient requires the
use of a fifty-six millimeter acetabular cup 12 (i.e. an acetabular
cup having an outer diameter of 56 mm), the reamer 50 utilized to
ream the patient's acetabulum 16 preferably has a cutting head
diameter of fifty-four millimeters (i.e. the outer diameter of the
cutting head is 54 mm). Such use of a smaller reamer 50 provides
numerous advantages. For example, it has been found that such use
of a slightly smaller reamer 50 creates a cavity 52 in the
acetabulum 16 which provides preferable amounts of insertion
resistance thereby firmly retaining the acetabular cup 12 upon
press fit thereof into the cavity 52 without requiring insertion
forces large enough to crack or otherwise break the acetabulum.
Hence, as shown in FIG. 4, use of the reamer 50 to ream the
acetabulum produces the cavity 52 having a radius R.sub.A. As
described above, the radius of the cutting head 54 of the reamer 50
is preferably slightly smaller than the radius R.sub.C of the
acetabular cup 12. Hence, the radius R.sub.A reamed into the cavity
52 of the acetabulum 16 is likewise slightly smaller than the
radius R.sub.C of the acetabular cup 12. Such a slight difference
in radius size provides for enhanced implantation properties. In
particular, as alluded to above, the acetabular cup 12 is
configured to be press fit into the reamed cavity 52 of the
acetabulum 16 without the use of bone cement. As such, the nearly
true hemispheric shape of the acetabular cup 12 provides for
constant contact with the reamed hemispherically shaped cavity 52
of the acetabulum 16 along the entire outer surface of the cup body
24.
Moreover, the presence of a slightly smaller radius R.sub.A of the
cavity 52 also causes a need for a slight increase in the insertion
force (i.e. the press fit) as the cup 12 is implanted in the
direction from the dome 28 to the annular rim 30. This slightly
increased resistance enhances the retention of the cup 12 when it
is press fit into the acetabulum 16. In addition, since the cavity
52 is reamed, for example, two (2) millimeters smaller in diameter
relative to the acetabular cup 12 (i.e. R.sub.A is 2 mm smaller
than R.sub.C), an approximately one millimeter difference is
created on each "side" of the annular rim 30.
Indeed, as shown in FIG. 6 according to one aspect thereof, a
"graduated" or slightly increasing press fit is created as the
acetabular cup 12 is implanted into the acetabulum 16.
Specifically, line-to-line contact exists between the cup 12 and
the cavity 52 in the area near the dome 28 of the cup 12. Hence,
the press fit of the cup 12 into the cavity 52 gradually increases
from approximately zero in the areas of such line-to-line contact
(i.e. the dome 28) to a press fit that equals a distance P on each
"side" of the cup 12 at the annular rim 30. In an exemplary case,
the distance P is one millimeter (1 mm) thereby creating an overall
press fit of two millimeters (2 mm) at the annular rim 30 of the
cup 12. This increasing press fit provides for a reliable (i.e.
stable) press fit of the acetabular cup 12 into the reamed cavity
52 thereby further enhancing the retention of the cup 12 in the
reamed cavity 52.
Moreover, as described above, since both the reamed cavity 52 and
the acetabular cup 12 are preferably configured as nearly true
hemispheres having similar sizes, the outer surface of the sidewall
26 of the cup 12 contacts the reamed hemispherically shaped cavity
52 of the acetabulum 16 along substantially all of the surface of
the cavity 52. Hence, the outer porous surface of the sidewall 26
of the acetabular cup 12 "scratches" or otherwise slightly abrades
substantially all of the wall surface of the cavity 52 as the cup
12 is press fit into the cavity 52. This slight abrading
facilitates bone ingrowth into the porous outer surface of the
acetabular cup 12.
As an alternative embodiment, the geometry of the acetabular cup 12
of the prosthetic hip assembly 10 of FIG. 1 may be modified as
follows. In particular, a modified acetabular cup 12A is shown in
FIG. 6A and is referred to by reference numeral 12A. The acetabular
cup 12A has 24a a body defining a dome having an apex 28a, an
annular rim 30a, and an outer sidewall 26a extending therebetween.
The annular rim 30a defines a plane PL and has a center point CP
lying in the plane. A radial distance between the center point CP
and the sidewall 26a gradually increases from the apex 28a to the
annular rim 30a. The radial distance from the center point CP to
the annular rim 30a defines a first given length, 1L. Further, the
radial distance from the center point CP to the apex 28a defines a
second given length, 2L. Note that the second given length is less
than the first given length. Further note that the first given
length is greater than a radius of curvature of the cutting head 54
of the reamer 50 (see FIG. 3). Moreover, the second given length is
approximately equal to the radius of curvature of the cutting head
54 of the reamer 50 (see FIG. 3). Preferably, 0.5
mm.ltoreq.1L-2L.ltoreq.2.0 mm. More preferably, 1L-2L is
approximately 1.0 mm.
Operation of the Present Invention
In operation, the prosthetic hip assembly 10 of the present
invention is utilized in the performance of either a total or
partial hip replacement procedure in order to provide an artificial
bearing surface for either a natural or artificial head portion of
the femur without the use of bone cement. As shown in FIG. 4, the
reamer 50 is first utilized to ream or otherwise cut the acetabulum
16 in order to form the hemispherically shaped cavity 52 therein.
In particular, the surgeon rotates the reamer 50 such that the
cutting projections 58 of the cutting head 54 engage and remove
bone material from the patient's acetabulum 16. As described above,
the radius of the cutting head 54 is preferably slightly smaller
than the radius R.sub.C of the acetabular cup 12. For example, the
reamer 50 utilized by the surgeon generally has a cutting radius
that is approximately one (1) millimeter smaller than the radius
R.sub.C of the acetabular cup 12 that is to be implanted. For
example, if the anatomy of the patient requires the use of a
fifty-six millimeter acetabular cup 12 (i.e. an acetabular cup
having an outer diameter of 56 mm), the reamer 50 utilized to ream
the patient's acetabulum 16 preferably has a cutting head diameter
of fifty-four millimeters (i.e. the outer diameter of the cutting
head is 54 mm).
Hence, as shown in FIG. 4, once the surgeon has utilized the reamer
50 to ream the acetabulum 16, the hemispherically shaped cavity 52
(having a radius R.sub.A) is formed. Thereafter, the surgeon
implants the acetabular cup 12 into the reamed acetabulum 16. In
particular, the acetabular cup 12 is press fit into the reamed
cavity 52 of the acetabulum 16 by the surgeon without the use of
bone cement. During such press fitting, the true hemispherical
shape of the acetabular cup 12 provides for constant contact with
the reamed hemispherically shaped cavity 52 of the acetabulum 16
along the entire outer surface of the cup body 24. Moreover, the
presence of a slightly smaller radius R.sub.A of the cavity 52 also
causes a need for a slight increase in the insertion force as the
cup 12 is implanted in the direction from the dome 28 to the
annular rim 30. This increases the retention of the cup 12 when it
is press fit into the acetabulum 16.
Moreover, since the cavity 52 is reamed, for example, two (2)
millimeters smaller in diameter relative to the acetabular cup 12
(i.e. R.sub.A is 2 mm smaller than R.sub.C), an approximately one
millimeter difference is created on each "side" of the annular rim
30. This slight difference provides for a reliable (i.e. stable)
press fit of the acetabular cup 12 into the reamed cavity 52.
In addition, during press fitting of the acetabular cup 12 into the
reamed cavity 52, the outer porous surface of the sidewall 26 of
the acetabular cup 12 "scratches" or otherwise slightly abrades
substantially the entire wall surface of the cavity 52. As
described above, this slight abrading facilitates bone ingrowth
into the porous outer surface of the acetabular cup 12. Moreover,
the similar configuration and size of the acetabular cup 12 and
reamed cavity 52 allows the cup 12 to be fully seated into a
position in which the annular rim 30 is substantially flush mounted
with the distal surface of the acetabulum 16 (see FIG. 5) without
requiring significant amounts of "estimating" by the surgeon during
reaming of the bone.
Once the acetabular cup has been press fit into the cavity 52
defined in the acetabulum 16, the bearing insert 14 is installed.
In particular, the bearing insert 14 may then be positioned in the
insert-receiving cavity 18 (see FIG. 1) defined in the acetabular
cup 12. As described above, the keying tabs 20 of the bearing
insert 14 are received into the corresponding keying slots 22
defined in the acetabular cup 12 to prevent rotation of the bearing
insert 14 relative to the acetabular cup 12. Once installed in such
a manner, the bearing insert 14 provides a desirable artificial
surface on which the artificial or natural head portion of the
femur may bear.
Hence, as described herein, the prosthetic hip assembly 10 of the
present invention provides numerous advantages over
heretofore-designed assemblies. For example, the prosthetic hip
assembly 10 of the present invention may be utilized to secure the
acetabular cup to the acetabulum without the use of bone cement.
Moreover, use of a constant radius, nearly true
hemispherically-shaped acetabular cup provides for enhanced
performance characteristics such as resistance to loosening and
instability since the configuration of the cup distributes loads
more evenly across the entire outer surface of the cup relative to
heretofore designed dual-geometry or bubble cups. In addition, the
use of a constant radius cup prevents the development of gaps near
the flared outer rim surfaces of heretofore-designed dual-geometry
or bubble cups. It is known that the presence of such gaps not only
prevents bone ingrowth, but also undesirably facilitates the
formation of lysis in the bone positioned near the rim of the
cup.
In addition, since both the reamer 50 and the acetabular cup 12 are
configured as nearly true hemispheres having similar sizes, the
surgeon is not required to estimate the approximate depth as to
when the acetabulum has been reamed deeply enough. This is a
significant improvement over the use of heretofore-designed
sub-hemispherical cups in which the surgeon must do such
estimating.
Moreover, since both the reamer (and hence the reamed cavity 52)
and the acetabular cup 12 are configured as nearly true hemispheres
having similar sizes, the outer surface of the sidewall 26 of the
cup 12 contacts the reamed hemispherically-shaped cavity 52 of the
acetabulum 16 along substantially all of the surface of the cavity
52. As described above, this feature causes the outer porous
surface of the sidewall 26 of the acetabular cup 12 to "scratch" or
otherwise slightly abrade substantially all of the wall surface of
the cavity 52 as the cup 12 is press fit into the cavity 52. Such
slight abrading advantageously facilitates bone ingrowth into the
porous outer surface of the acetabular cup 12.
Further, the similar configuration and size of the acetabular cup
12 and the reamed cavity 52 also allows the cup 12 to be fully
seated into a position in which the annular rim 30 is substantially
flush with the distal surface of the acetabulum 16 (see FIG. 5)
without requiring significant amounts of "estimating" by the
surgeon during reaming of the bone.
Moreover, use of a reamer that is slightly smaller than the
acetabular cup provides preferable amounts of resistance thereby
firmly retaining the acetabular cup upon press fit thereof into the
cavity without requiring insertion forces large enough to crack or
otherwise break the acetabulum.
While the invention has been illustrated and described in detail in
the drawings and foregoing description, such an illustration and
description is to be considered as exemplary and not restrictive in
character, it being understood that only the preferred embodiment
has been shown and described and that all changes and modifications
that come within the spirit of the invention are desired to be
protected.
There is a plurality of advantages of the present invention arising
from the various features of the prosthetic hip assembly and
associated method described herein. It will be noted that
alternative embodiments of the prosthetic hip assembly and
associated method of the present invention may not include all of
the features described yet still benefit from at least some of the
advantages of such features. Those of ordinary skill in the art may
readily devise their own implementations of a prosthetic hip
assembly and associated method that incorporate one or more of the
features of the present invention and fall within the spirit and
scope of the present invention as defined by the appended
claims.
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